This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Lignin is the most abundant terrestrial biopolymer after cellulose. Lignin precursors are exclusively methylated at their meta-positions (i.e., 3/5-OH) of the phenyl rings, and are precluded from the substitution at the para-hydroxyl position;in fact, the para-hydroxyls of monolignols are proposed to be critically important for generating oxidative radicals, and cross-linking lignin units. Therefore, methylation of the para-hydroxyl (i.e., 4-OH) position of monolignol should interfere with the synthesis of the lignin polymer. To test this hypothesis, we propose a structure-based protein engineering approach, to investigate the molecular mechanisms of regiospecific O-methylation of lignin precursors and natural phenylpropenes, thereby, to create a set of novel monolignol 4-O-methyltransferases that will introduce the non-natural para-methoxyl monolignols in planta. Specifically, we will explore the structure-function relationships of two types of functionally distinct but structurally related enzymes, viz., phenylpropene 4-O-methyltransferase and lignin 3/5-O-methyltransferase, to understand the distinct regiospecific methylation and substrate discrimination. The resulting information will be used to create comprehensive libraries of the variants of lignin 3/5-O-methyltransferase and phenylpropene 4-O-methyltransferase, employing both the approaches of structure-based rational design and the directed protein evolution. With high-throughput colorimetric screening, we will select a range of novel variants able to efficiently methylate the para-hydroxyl of the different monolignols. Information from these studies will provide a scientific underpinning for the rational manipulation of lignin biosynthesis to improve the efficiency of biofuel production, and thus contribute to decreasing our dependence on petrochemical fuels.